1. Technical Field
This invention relates generally to batteries for portable communications devices, and more specifically to batteries for portable communications devices that help reduce the macroscopic SAR level for a given power transmission level.
2. Background Art
All wireless communications devices, be they cellular phones, pagers, radios or other devices, include at least two components: a battery and a radio-frequency (RF) communications module. The battery, which is often rechargeable, provides portability for the device. The RF module allows the device to communicate with networks and other devices through radio waves transmitted and received with the device""s antenna.
When the RF module is transmitting, a power amplifier magnifies an electronic signal and then broadcasts this modulated energy into the air through the antenna. Some of this energy travels through free space to the target destination. Some of this energy is dissipated as heat within the device. Some of the energy is absorbed by objects near the device, including the user""s body.
Both governmental and standards organizations have set limits for maximum levels of power that may be absorbed by a person using the device. Specifically, the Federal Communications Commission (FCC) has set forth rules and regulations relating to RF exposure from communications devices. The rules are based upon input from independent scientific organizations. In particular, these regulations place limits on the SAR, or Specific Absorption Rate. The SAR is a measurement of RF energy absorbed by the body when using a communications device. When a communications device is operating within the FCC SAR limits, scientific panels, government agencies, public health authorities and standards bodies, including the U.S. Food and Drug Administration and the World Health Organization, all agree that electronic devices pose no hazards to users.
The SARs for various manufacturers"" phones can be different. This is due in part to the fact that SAR is a function of many different parameters, including device geometry, transmission frequency, amplifier power level, antenna location, etc. Engineers often refer to SAR issues as xe2x80x9cblack magicxe2x80x9d because it is very difficult to derive mathematical equations that accurately predict SAR due to the many parasitic components in a phone that affect high-frequency RF transmission. One thing is certain, however: lower SAR device configurations are preferable because they both improve performance and extend battery life. Consider the following hypothetical example.
Imagine that the SAR limit is, for discussion purposes, 10. If two phones have different xe2x80x9cSAR profilesxe2x80x9d, the power levels of the power amplifiers must be set differently in each phone. If one phone has a xe2x80x9cnoisyxe2x80x9d SAR profile, the power levels in that phone must be set lower that that in a phone with a xe2x80x9cquietxe2x80x9d SAR profile to achieve the same macroscopic limit of 10. Having a lower power level is disadvantageous because the maximum distance from a base station within which the phone can communicate is reduced. Additionally, as the quiet SAR profile phone will generally operate in the field at less than maximum power, battery life is extended.
There is thus a need for an apparatus that reduces the macroscopic SAR level of communications devices.